The present invention relates to a method for setting down or taking up a load from or upon a loading location by means of a crane, wherein the crane or the loading location is subjected to substantially irregular vertical movements which change the distance between the hoisting hook of the crane and the loading location.
The invention also relates to an apparatus for carrying out the method.
For vessels which are on the sea or in the air, it may often in stormy weather be difficult to carry out loading and unloading operations without damaging the load. Semi-submerged drilling platforms or other seagoing vessels of the ship type which are used in work in open sea obtain, for example, a vertical movement due to waves or rolling sea. Such a movement may entail that a load which is to be put down outside the vessel by a crane thereon, or a load which is to be put down on a vessel from another seagoing vessel or, for example a helicopter, meets the base with such a force that damage is effected on the load and the base. In connection with large and heavy loads such movement may be very critical, which entails that the loading and unloading work can be carried out only during relatively calm weather conditions. This may mean a prolonged waiting time for carrying out the planned work, which in turn may raise the cost of the subject project in dislocating the progress schedule thereof.
In connection with semi-submerged drilling platforms there is known a heave compensation technique which is used during drilling operations. Here, the pressure exerted by the tip of the drill bit against the bottom of the drilling well is sensed. The pressure, which substantially is made up of the weight of the drilling string, is kept at a suitable level by controllable counteracting means. These respond in pace with the pressure changes which are due either to the increased length of the drilling string or the heaving motions of the drilling platform. In connection with heave compensation for hoisting crances, the pressure changes sensed are believed to be too small for affording a rapid and sufficiently accurate result which meets the requirements for a sterling heave compensation which are demanded for hoisting cranes on vessels at sea.
Another so-called heave compensating system is known in connection with cranes used on vessels working in open sea. The system here comprises a heave-energy absorbing device which is suspended in the hoisting hook and constitutes an extension thereof. When the load is hooked onto the hoisting hook via the energy absorbing device, which is designed with a telescope part consisting of a series of cylinders, the device will follow the wave motions or the heave motions approximately 3 to 4 times whereafter, at the top of such a motion, it will raise the load by means of the stored energy. The load will then be raised so high that it clears the top of the next wave.
Such a device is, however, only advantageous when a load is to be lifted, for example from an auxiliary vessel to a drilling platform. In landing a load on a base which moves relative to the crane, the system is without value.